The 80 Ms follow-up of the X-ray afterglow of GRB 130427A challenges the standard forward shock model

TL;DR

This study presents an 80-million-second X-ray afterglow light-curve of GRB 130427A, revealing a simple power-law decay that challenges the standard forward shock model's ability to explain such long-term behavior.

Contribution

It provides the longest X-ray afterglow observation of a GRB, critically testing the standard forward shock model and highlighting its limitations in explaining extended afterglow decay.

Findings

X-ray afterglow follows a consistent power-law decay over 80 million seconds

Standard forward shock model struggles to fit the observed data

Requires unrealistic physical parameters to match the observations

Abstract

GRB 130427A was the brightest gamma-ray burst detected in the last 30 years. With an equivalent isotropic energy output of $8.5\times10^{53}$ erg and redshift $z=0.34$, it uniquely combined very high energetics with a relative proximity to Earth. As a consequence, its X-ray afterglow has been detected by sensitive X-ray observatories such as XMM-Newton and Chandra for a record-breaking baseline longer than 80 million seconds. We present the X-ray light-curve of this event over such an interval. The light-curve shows a simple power-law decay with a slope $\alpha = 1.309 \pm 0.007$ over more than three decades in time (47 ks - 83 Ms). We discuss the consequences of this result for a few models proposed so far to interpret GRB 130427A, and more in general the significance of this outcome in the context of the standard forward shock model. We find that this model has difficulty in explaining our data, in both cases of constant density and stellar wind circumburst media, and requires far-fetched values for the physical parameters involved.